i6 



CORALS 



find many examples of organisms which may or may not be 

 called polyps according to the inclination of different authors. 

 But in general terms a polyp is an animal which is 

 sedentary in habit and more or less C3'lindrical in shape, 

 which possesses a mouth, surrounded by a crown of tentacles, 

 and an alimentary canal or a body cavity in which food is 

 digested. 



There are many polyps which are solitary, but more 

 generally they build up, by budding or by division, colonies 

 of polyps in organic continuity with one another. If we 



take an example of 

 a solitary polyp, the 

 coral Caryophyllia 

 (Fig. i), we can see, 

 when it is fully ex- 

 panded, that it pos- 

 sesses a mouth placed 

 in the centre of a 

 disc surrounded by a 

 single ring of ten- 

 tacles. In a colony 

 of polj'ps, as is shown 

 in the diagram of an 

 Alcj'onarian (Fig. 44) , 

 we see a number of 

 polyps connected to- 

 gether in a common 

 fleshy substance (the 

 coenenchym) by a system of canals (coenosarcal canals). 

 Sometimes we find that the polyps in such a colony are all 

 alike in structure ; in other cases we find they are of two 

 kinds, as in the diagram, when the}' are called dimorphic 

 colonies. In the dimorphic colonies the two kinds of polyps 

 perform different physiological functions and show different 

 structural characters in adaptation to the performance of 

 those functions. In such cases the word zooid is used instead 

 of polyp, with a prefix to indicate in some way the functions 

 it performs {e.g. Autozooid and Siphonozooid in Fig. 44). 

 The reasons for calling the polyps Animals can now be 



Fig. I. — A fully e,\paiided Caryophyllia polyp, 

 showing the slit-like mouth in the centre and the 

 ring of capitate tentacles surrounding the oral disc. 

 (After de Lacaze-Duthiers.) x li. 



